Higher Order Reflections Research Articles

Gaussian distribution-based randomised image method

Sweeping echo is an acoustical phenomenon present in perfectly rectangular rooms that is characterised by a continuous increase in pitch arising from the concurrent arrival of multiple higher-order reflections. Even though the room acoustic simulation software predicts their presence, in most rectangular rooms sweeping echoes can hardly be perceived due to the existence of small irregularities in the room geometry. In this paper, the image sources' position randomisation model developed within the randomised image method framework with which the incongruence between the original image method results and in situ measurements can successfully be reduced is replaced with a Gaussian distribution model. The results indicate that with the proposed model an improvement in the stability of the room transfer function across realisations can be achieved.

High-transmission spectrometer for rapid resonant inelastic soft X-ray scattering (rRIXS) maps.

The design and first results of a high-transmission soft X-ray spectrometer operated at the X-SPEC double-undulator beamline of the KIT Light Source are presented. As a unique feature, particular emphasis was placed on optimizing the spectrometer transmission by maximizing the solid angle and the efficiencies of spectrometer gratings and detector. A CMOS detector, optimized for soft X-rays, allows for quantum efficiencies of 90% or above over the full energy range of the spectrometer, while simultaneously offering short readout times. Combining an optimized control system at the X-SPEC beamline with continuous energy scans (as opposed to step scans), the high transmission of the spectrometer, and the fast readout of the CMOS camera, enable the collection of entire rapid resonant inelastic soft X-ray scattering maps in less than 1 min. Series of spectra at a fixed energy can be taken with a frequency of up to 5 Hz. Furthermore, the use of higher-order reflections allows a very wide energy range (45 to 2000 eV) to be covered with only two blazed gratings, while keeping the efficiency high and the resolving power E/ΔE above 1500 and 3000 with low- and high-energy gratings, respectively.

A new rhombohedral phase and its 48 variants in β titanium alloy

A new rhombohedral phase (termed R′) in a solution−aging-treated titanium alloy (Ti−4.5Al−6.5Mo−2Cr− 2Nb−1V−1Sn−1Zr, wt.%) was identified. Its accurate Bravais lattice parameters were determined by a novel unit cell reconstruction method based on conventional selected-area electron diffraction (SAED) technique. The orientation relationship between R′ phase and BCC phase was revealed. The results show that the R′ phase is found to have 48 crystallographically equivalent variants, resulting in rather complicated SAED patterns with high-order reflections. A series of in-situ SAED patterns were taken along both low- and high-index zone axes, and all weak and strong reflections arising from the 48 variants were properly explained and directly assigned with self-consistent Miller indices, confirming the presence of the rhombohedral phase. Additionally, some criteria were also proposed for evaluating the indexed results, which together with the Bravais lattice reconstruction method shed light on the microstructure characterization of even unknown phases in other alloys.

Adjoint Algorithm Design of Selective Mode Reflecting Metastructure for BAL Applications.

Broad-area lasers (BALs) have found applications in a variety of crucial fields on account of their high output power and high energy transfer efficiency. However, they suffer from poor spatial beam quality due to multi-mode behavior along the waveguide transverse direction. In this paper, we propose a novel metasurface waveguide structure acting as a transverse mode selective back-reflector for BALs. In order to effectively inverse design such a structure, a digital adjoint algorithm is introduced to adapt the considerably large design area and the high degree of freedom. As a proof of the concept, a device structure with a design area of 40 × 20 μm2 is investigated. The simulation results exhibit high fundamental mode reflection (above 90%), while higher-order transverse mode reflections are suppressed below 0.2%. This is, to our knowledge, the largest device structure designed based on the inverse method. We exploited such a device and the method and further investigated the device's robustness and feasibility of the inverse method. The results are elaborately discussed.

Narrowband Mg/SiC multilayer mirror working as high-harmonic selector at 30.4 nm wavelength

Multilayers (MLs) are a good choice for high-harmonic generation (HHG) sources for selecting single-wavelength radiation. MLs working around 30.4 nm with a significantly reduced bandwidth based on high Bragg order reflection have been designed and fabricated. The narrowband MLs were characterized using grazing incidence x-ray reflectometry, extreme ultraviolet reflectivity, and intrinsic stress measurements. The results indicate that Mg/SiC MLs designed with 3rd Bragg order have narrowest bandwidth of 0.71 nm (full width half maximum) with a spectral resolution (λ/Δλ) of 42, and reflectance of 30% under near normal incidence geometry. Based on these results, the simulation of narrowband MLs working at large incident angles demonstrates that 3rd Bragg order ML has a better spectral selectivity compared with 1st Bragg order ML for HHG sources.

Analyzing the Practice of Critical Thinking Skills for Islamic Education Students in the International Campus

Critical thinking is one of the most important goals of any educational system. Previous research has indicated that Malaysian educational institutions are still unable to meet the goals of instructional pedagogy in the classroom, particularly in terms of improving critical thinking abilities. The purpose of this study is to examine the practice of critical thinking skills among undergraduate students from the perspective of faculty members. This study is qualitative in nature since it focuses on individuals’ experiences and impressions of a specific programs from their own viewpoints. Data was gathered through interviews with faculty members who teach Islamic Education undergraduate students at the Kulliyyah of Education (KOED), International Islamic University Malaysia (IIUM). The findings of the study revealed that practicing critical thinking abilities improved the students’ learning experiences. According to the informants’ response, a critical thinking skill is the student's ability to gather knowledge and explain his or her thoughts in order to solve difficulties. Similarly, students may exercise critical thinking abilities by developing higher-order thinking skills, problem-solving skills, reflection skills, and synthesis skills. Moreover, the study recommends further investigations on the practice of critical thinking skills not only for undergraduate students but also for other postgraduate students, particularly at the Kulliyyah of Education (KOED), IIUM. It also suggests that management educational institutions offer intensive programs to help students strengthen their critical thinking abilities.

A 3D complex urban sound propagation benchmark case

Predicting sound propagation in the dense urban fabric is challenging as it involves multiple and higher-order reflections, both specular and diffuse in nature, diffraction around complex objects like building edges and roofs, and atmospheric effects especially relevant at non-directly exposed locations. Over the last decades, there has been a continuing interest in enclosed courtyards, a micro-environment with the potential to offer human restoration in a busy city. Also roofs are increasingly being designed as outdoor spaces. The sound exposure in such zones could be especially challenging to predict. To gain more understanding in the related physics and accuracy of prediction schemes, a 3D urban benchmark case is presented, consisting of 4 building blocks with a fully symmetric geometry. Receivers and acoustic sources are positioned in the streets, on the roofs, and in the courtyards. Simulations with the reference finite-difference time-domain technique show pronounced differences in the time responses, and allow assessing the impact of including (geometrical) façade details. Additional simulations show the impact of openings from within the streets towards the courtyards. The simulation results allow checking the accuracy of sound propagation models and their simplifications.

High-power narrow spectral width 975 nm semiconductor laser with high process compatibility achieved by high-order gratings.

We present broad-area semiconductor lasers with a 100th-order high-order distributed Bragg reflection (HO-DBR) grating structure, fabricated with the conventional UV lithography techniques. Based on the finite-difference time-domain (FDTD) algorithm, a maximum output power of 10.5 W, emitting around 975 nm with a spectral width of less than 0.5 nm FWHM has been achieved. This method provides insights for reducing the manufacturing costs of the high-power narrow spectral width DBR lasers.

Amplitude-Constrained Constellation and Reflection Pattern Designs for Directional Backscatter Communications Using Programmable Metasurface

The large scale reflector array of programmable metasurfaces is capable of increasing the power efficiency of backscatter communications via passive beamforming and thus has the potential to revolutionize the low-data-rate nature of backscatter communications. In this paper, we propose to design the power-efficient higher-order constellation and reflection pattern under the amplitude constraint brought by backscatter communications. For the constellation design, we adopt the amplitude and phase-shift keying (APSK) constellation and optimize the parameters of APSK such as ring number, ring radius, and inter-ring phase difference. Specifically, we derive closed-form solutions to the optimal ring radius and inter-ring phase difference for an arbitrary modulation order in the decomposed subproblems. For the reflection pattern design, we propose to optimize the passive beamforming vector by solving a multi-objective optimization problem that maximizes reflection power and guarantees beam homogenization within the interested angle range. To solve the problem, we propose a constant-modulus power iteration method, which is proven to be monotonically increasing, to maximize the objective function in each iteration. Numerical results show that the proposed APSK constellation design and reflection pattern design outperform the existing modulation and beam pattern designs in programmable metasurface enabled backscatter communications.

On the origin of diffuse intensities in fcc electron diffraction patterns.

Interpreting diffuse intensities in electron diffraction patterns can be challenging in samples with high atomic-level complexity, as often is the case with multi-principal element alloys. For example, diffuse intensities in electron diffraction patterns from simple face-centred cubic (fcc) and related alloys have been attributed to short-range order1, medium-range order2 or a variety of different {111} planar defects, including thin twins3, thin hexagonal close-packed layers4, relrod spiking5 and incomplete ABC stacking6. Here we demonstrate that many of these diffuse intensities, including [Formula: see text]{422} and [Formula: see text]{311} in ⟨111⟩ and ⟨112⟩ selected area diffraction patterns, respectively, are due to reflections from higher-order Laue zones. We show similar features along many different zone axes in a wide range of simple fcc materials, including CdTe, pure Ni and pure Al. Using electron diffraction theory, we explain these intensities and show that our calculated intensities of projected higher-order Laue zone reflections as a function of deviation from their Bragg conditions match well with the observed intensities, proving that these intensities are universal in these fcc materials. Finally, we provide a framework for determining the nature and location of diffuse intensities that could indicate the presence of short-range order or medium-range order.

A HYBRID STRUCTURE OF FREQUENCY SELECTIVE SURFACE AND MAGNETIC ABSORBING SALISBURY SCREEN FOR WIDEBAND RADAR CROSS SECTION REDUCTION

This article presents a novel hybrid physical mechanism that combines frequency selective surface (FSS) and Salisbury-type absorption to create a structure that can reduce the radar cross section (RCS) over a wide frequency range. The proposed design integrates a lossy sheet composed of a magnetic absorbing material (MAM) with a traditional Salisbury screen and a FSS of a T-ring resonator. The Salisbury screen effectively absorbs incoming waves when illuminated by a plane wave with normal incidence. Furthermore, the integration of a T-ring resonator generates high-order reflection beams, thereby enabling the selection of absorption band. Simulation results demonstrate that the proposed structure enables tunable RCS reduction between 2 and 18 GHz under various conditions with normal incidence. The combination of broadband absorption mechanism with frequency selective resonance allows to construct a perfect structure both in terms of bandwidth and frequency selection of absorption.

Reflection and refraction of higher-order Hermite-Gaussian beams: a vector wave analysis.

Hermite-Gaussian beams, as a typical kind of higher-order mode laser beams, have attracted intensive attention because of their interesting properties and potential applications. In this paper, a full vector wave analysis of the higher-order Hermite-Gaussian beams upon reflection and refraction is reported. The explicit analytical expressions for the electric and magnetic field components of the reflected and refracted Hermite-Gaussian beams are derived with the aid of angular spectrum representation and vector potential in the Lorenz gauge. Based on the derived analytical expressions, local field distributions of higher-order Hermite-Gaussian beams reflection and refraction at a plane interface between air and BK7 glass are displayed and analyzed.

Superfluid Signatures in a Dissipative Quantum Point Contact.

We measure superfluid transport of strongly interacting fermionic lithium atoms through a quantum point contact with local, spin-dependent particle loss. We observe that the characteristic non-Ohmic superfluid transport enabled by high-order multiple Andreev reflections transitions into an excess Ohmic current as the dissipation strength exceeds the superfluid gap. We develop a model with mean-field reservoirs connected via tunneling to a dissipative site. Our calculations in the Keldysh formalism reproduce the observed nonequilibrium particle current, yet do not fully explain the observed loss rate or spin current.

EM Scattering of A Target above Canyon/Valley Environment Based on Composite Rough Surface Modeling Method and Modified SBR-FBSSA Algorithm

The composite electromagnetic (EM) scattering characteristics from a target above a canyon/valley environment are significant. Aiming to acquire the composite EM scattering efficiently and accurately, the framework of the canyon/valley environment modeling method and modified shooting and bouncing rays (SBR) hybrid with facet-based small slope approximation (FBSSA) algorithm is investigated. Firstly, the canyon/valley environment containing two slopes and a bottom modeling method is proposed. Then, considering the environment’s roughness, the modified SBR algorithm introduced by the high-order reflection model is proposed. Combined with the FBSSA, the modified SBR-FBSSA algorithm is an efficient and accurate method to predict composite EM scattering based on numerical verification. Finally, the effects of different surface types, roughness, slope angles, and incident-pitch and azimuth angles on the composite EM scattering characteristics are further analyzed. The work presented in this article provides a way to study the composite EM scattering from a target above the canyon/valley environment. Meanwhile, the complex scattering mechanism is revealed, and some valuable conclusions are put forward based on the physical phenomena.

Spatial Interpolation of Early Room Impulse Responses Using Equivalent Source method based on Grouped Image Sources

The Room Impulse Response (RIR), which indicates the propagation characteristics of sound, is used in various acoustical applications. In particular, multi-point measurement of RIRs is important for applications that require spatial information, and measurement of these is difficult. Previously, spatial interpolation methods of RIRs based on physical models have been proposed. Especially, the sparse Equivalent Source Method (ESM) with Image Source Method (ISM) acheive the spatial interpolation of the early reflected sounds with higher accuracy at high frequency. However, this method has a problem that when the order of reflected sounds to be estimated increases, it becomes difficult to find the optimal solutions because the number of bases increases. In this study, to reduce the number of bases in each optimization problem, we proposed a sparse ESM with the image sources grouped by the arrival times of reflected sounds. The optimization problem for each group of image sources is individually solved. In the simulation experiment, it is revealed that the proposed method can estimate higher-order reflections with higher accuracy compared to the conventional method. In addition, we found that the estimated region with high accuracy became broader.

2nd and 3rd order spectral energy corrections with penumbral de-blurring methodology for opacity platform used on the National Ignition Facility.

The Opacity Spectrometer (OpSpec) used in the National Ignition Facility's opacity experiments measures x-ray spectra from 0.9 to 2.1keV from the different experimental regions: the backlight source, emission source, and the absorption region with the transmission calculated from these regions. The OpSpec designs have gone through several iterations to help improve the signal-to-noise ratio, remove alternate crystal plane reflections, and improve spectral resolution, which helps to increase the validity of the opacity measurements. However, the source spans well outside the current working spectral range, and higher-order reflections are intrinsic to the crystal, which increases the overall signal seen in the data regions. The recorded data are the convolution of 1st order transmission, higher-order reflections, and the penumbra blurring. This work represents the details for deconvolving the 2nd and 3rd order spectral energy corrections with a penumbral de-blurring to correct the relative measurement of x-ray intensity of different spectral energies and further analysis of datasets relevant to the opacity experiments.

Fabrication of parallel integrated few-mode fiber Bragg grating with high-order mode self-coupling reflection

In this paper, a parallel integrated few-mode fiber Bragg grating (FM-FBG) with high-order mode self-coupling reflection is produced by the femtosecond laser point-by-point method. The FM-FBGs using fixed parameters are written radially in the same core repeatedly to increase the refractive index modulation region so that the proposed high-order modes can obtain high coupling coefficients. Four parallel integrated FM-FBGs with different periods were made and tested. Precisely, LP01, LP11, LP21, and LP02 modes all achieve self-coupling reflection at the same wavelength of 1550 nm within the full width at half maximum (FWHM) of their self-coupling reflection peaks. Besides, the effects of grating length, grating position, laser energy, and the grating order on the self-coupling reflectivity of different modes were investigated. From the experimental results, it can be seen that the self-coupling reflectivity of the four modes at the wavelength of 1550 nm reaches 94.07%, 69.52%, 68.81%, and 63.52%, respectively.

The Role of the Prefrontal Cortex in Conscious Perception: The Localist Perspective

In recent years the role of the prefrontal cortex (PFC) in conscious perception has acquired great interest since it became a pivotal issue distinguishing among prevailing neuronal theories of human consciousness. One can identify three major, and conflicting, views of this role. The globalist view proposes that the PFC is a major hub in a global workspace whose activation is a critical component for any conscious experience. The higher-order thought theory argues that the PFC has a more specialized role underlying higher-order reflection or evaluation, proposed to be an essential element of consciousness. By contrast, the localist view argues that conscious content is linked to localized activations in content-specific cortical areas with no privileged role assigned to the prefrontal cortex in conscious experience in general. According to the localist view, just as posterior cortical areas underlie the conscious experience of visual perceptions, so does the prefrontal cortex underlie the conscious experience of specific categories of conscious contents such as reporting, thinking, speech, and introspection. Here I will review experimental evidence derived from human imaging and recordings, cortical lesions, and direct electrical stimulation in awake patients. Findings from these

Toolbox for modeling frequency-domain photothermal experiments on multilayers

A general expression (master equation, ME) is derived for the thermal impedance in photothermal experiments in a multilayer system, based on one-dimensional (1D) periodic heat diffusion. The ME in a compact form benefits from newly defined generalized, higher-order thermal reflection and transmission coefficients. The modeled system comprises seven layers among which a semitransparent sample and a transducer that integrates the temperature field within it (e.g., a pyroelectric sensor). The ME can be adapted to various experimental cell structures used in photopyroelectric, photoacoustic, photothermal radiometry, or thermoreflectance methods using volume- or surface-temperature detection, in view of spectroscopic applications or thermophysical properties determination. The derivation of special cases is facilitated by applying simple contraction rules to dimensionless quantities. Modeling multiple heat sources in the system is done by superposition of individual solutions. The possible extension of the 1D model to 2D geometry is demonstrated, in general, and practical criteria are discussed.

High-Order Pulse-Echo Ultrasound

Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wave front with the imaged object is traditionally regarded as an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. We introduce high-order reflection pulse-echo (HOPE) ultrasound, a method that leverages high-order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve submicrometer features by breaking through the sampling limit. The major contrast enhancement of the high reflection orders allowed defects within materials invisible to conventional scanning acoustic microscopy to be revealed. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in biomedical imaging, nondestructive testing, and other acoustic sensing applications.